Electrical indicating instruments

ABSTRACT

An indicating instrument is described which displays the value of some driving or control signal on a viewing screen as a sharp, bright image which is devoid of noticeable light-dispersion effects, such as image shadowing or rainbowing. This result is accomplished by focusing a concentrated light beam onto the path of movement of the image-forming element which takes the more specific form of a movable transparent vane. The diverging vane image is focused onto the viewing screen by means of a concave mirror having its concave reflecting surface spaced from, and facing, the back surface of the viewing screen. The vane is caused to move in the prescribed path in response to the signal applied to the instrument. The concave mirror is preferably of spherical or aspheric shape and serves as an efficient collector and reflector of incident light rays. Also disclosed is a panelmountable indicating instrument of overall rectangular shape and a flat and relatively thin edgewise appearance made possible by an optical system which provides vane images having the abovedescribed characteristics.

United States Patent [72] Inventors Antonio S. Paiva Newark; Justin V.Paulauskas, Elizabeth, both of, NJ. [21] Appl. No. 751,436 [22] FiledAug. 9, 1968 [45] Patented Aug. 17, 1971 [73] Assignee WestonInstruments, Inc.

Newark, NJ.

[54] ELECTRICAL INDICATING INSTRUMENTS 6 Claims, 3 Drawing Figs.

[52] US. Cl 353/40, 353/14, 353/78 [51] Int. Cl ..G03b 21/10 [50] Fieldof Search 353/40, 41, 14, 78,47, 50, 51,98, 99; l16/114.12

[56] References Cited UNITED STATES PATENTS 2,831,253 4/1958 Hodges33/147 (E) 3,189,111 6/1965 Ast 353/41 X 975,949 11/1910 Henwood 353/401,740,130 12/1929 Von Vosset al... 116/1 14.12

1,811,770 6/1931 Wheeler 353/50 X 1,893,421 1/1933 Latzko etal. 353/402,191,045 2/1940 Slayton 353/14 2,570,621 10/1951 Williams 353/40FOREIGN PATENTS 563,567 8/1944 Great Britain 353/40 635,170 4/1950 GreatBritain 353/40 Primary Examiner-Harry N. l-laroian Att0rneysWilliam R.Sherman, Stewart F Moore and Jerry M. Presson ABSTRACT: An indicatinginstrument is described which displays the value of some driving orcontrol signal on a viewing screen as a sharp, bright image which isdevoid of noticeable light-dispersion effects, such as image shadowingor rainbowing. This result is accomplished by focusing a concentratedlight beam onto the path of movement of the image-forming element whichtakes the more specific form of a movable transparent vane. Thediverging vane image is focused onto the viewing screen by means of aconcave mirror having its concave reflecting surface spaced from, andfacing, the back surface of the viewing screen. The vane is caused tomove in the prescribed path in response to the signal applied to theinstrument. The concave mirror is preferably of spherical or asphericshape and serves as an efficient collector and reflector of incidentlight rays. Also disclosed is a panel-mountable indicating instrument ofoverall rectangular shape and a flat and relatively thin edgewiseappearance made possible by an optical system which provides vane imageshaving the abovedescribed characteristics.

ELECTRICAL INDICATIN G INSTRUMENTS screen.

Prior art instruments of this general type conventionaily employ asource of practically white light, such as an incandescent lamp, whichilluminates a light-transmitting viewing screen. The screen may bear ascale graduated in terms of one or-more parameters of the measuredsignal or the screen may be completely devoid of indicium and insteadserve as a medium for displaying images of indicia projected onto thescreen. In such instruments, use is made of a movable thin vane or mask,generally of rectangular cross-sectional shape, the vane being displacedin response to the signal to intercept that portion of the lightreceived by the screen which represents the measured signal value or thecomplement thereof.

Transparent, colored vanes are oftentimes preferred to opaque vanesbecause the former type of vanes provide a recognizable color contrastbetween the vane image which is displayed on the viewing screen andbackground screen illumination provided, for example, by unfiltered ordifferently filtered light from the incandescent lamp. Transparent vanesare often used as the medium which bears opaque indicia such asnumerals, letters, or other characters for projection onto the viewingscreen. For some applications it may be desired to project the vaneconfiguration or shape onto the screen, with or without indicia.

' ln instruments of this type it is manifestly requisite that the imagewhich is projected onto the screen be as bright as possible and sharp,that is, devoid of all light-dispersion effects which typically causeedge shadowing and/or rainbowing" of the displayed image. I

A typical prior art approach to obtain, for example, a movingrectangular bar or band of colored light visible from the front screensurface is to shadow the illumination of the back screen surface with arectangular, transparent vane of some suitable color. However, theproblem with this approach is that the light source which provides thescreen and vane illumination is typically obtained, for various obviousreasons, from an incandescent lamp formed with a filament having arelatively large light radiating surface area. The lamp is constrainedto a position relatively near the vane by practical limitations placedon the maximum permissible size of the instrument casing. The light raysemanating from a nearby light source of some relatively large area (ascontrasted to a virtual point source of light located at some positionremote from the vane) are not parallel light rays and consequentlyproduce noticeable diffraction and other light-dispersion-effectsadjacent the edge portion of a vane positioned in these nonparallellight rays. These light-dispersion effects appear as a noticeableshadowing or blurring of the projected image particularly adjacent theleading edge of the image.

In other types of prior art instruments of the specified type use ismade of a transparent vane bearing opaque indicia, or vice versa. Insuch instruments the indicium corresponding to the value of the measuredsignal is typically projected onto a screen by an optical projectionsystem comprised of one or more projecting lenses. However, like aprism, a projection lens also has an index of refraction and thereforemay impart chromatic or spherical aberrations to the practically whitelight received from the incandescent lamp. The spreading out ordispersion of the light emerging from the projecting lens intoessentially the basic six colors causes visually noticeable spectrallines or rainbowing of the projected image. Accordingly, such prior artinstruments sometimes display images which lack brightness andcoherency.

According to one embodiment of this invention, there is provided anindicating instrument suitable for mounting on an instrument panel whichdisplays for viewing the value of a measured signal on an elongatedviewing screen. The value is displayed as an image, which is extremelybright and sharply defined and devoid of disturbinglight-dispersion'efi'ects. This result is accomplished by forming andprojecting, by means of a converging lens system, a convergent lightbeam across a prescribed path of displacement of an image-formingelement, referred to herein as a vane," and onto the reflecting surfaceof a concave mirror having an elongated mirror surface of spherical oraspheric shape. The mirror is positioned so that the mirror surfacethereof is spaced from and faces a lengthwise section of the backsurface of the viewing screen which is to be illuminated thereby. Thevane is caused to move in the prescribed path in response to themeasured signal. The focal points of the converging lens system and theconcave mirror are practically in the same plane as that prescribed forvane travel. The portion of the vane which intercepts the concentratedlight beam is transmitted as a bright, sharply defined image onto themirror surface. The mirror collects and reflects incident light rayswith high optical efficiency and accordingly is able to focus a sharp,brilliant image of the vane portion onto the desired area of the viewingscreen without accompanying light-dispersion effects.

The vane may be formed as a thin strip of film or similar device. Thevane may be completely opaque, completely transparent, or partiallyopaque and partially transparent, one obvious example of the latterbeing a transparent vane bearing opaque indicia. As exemplary of oneembodiment of this invention, the vane is disclosed as a rectangular,transparent filter having a color selected to provide an easilydetectable color contrast to the background screen illumination. Theprojected image portion of the vane is thus displayed as a sharp andbrightly contrasting rectangular light bar.

For better understanding of the present invention, together with otherand further objects thereof, reference may be had to the followingdescription taken in connection with the accompanying drawings, thescope of the invention being pointed out in the appended claims.Referring to the drawings:

FIG. 1 is a plan view of one embodiment of the invention with a topcovering plate removed for purposes of clarity;

FIG. 2 is a cross-sectional view taken on line 2-2 of FIG. 1 with thetop plate mounted on the instrument; and

FIG. 3 is a front end 'view of FIG. 1 illustrating the type of light barwhich is obtainable with the instrument of this invention.

Referring to FIGS. 1 and 2, numeral 10 designates an instrumentconstructed in aecordancewith the principles of this invention. Theoptical system for the instrument is mounted within a rectangular casingwhich is sealed against the entry of ambient light, dust and otherextraneous matter which might deleteriously affect the operation of theoptical system. The internal wall of the casing may be coated with asuitable lightabsorbing material to absorb extraneous light rays in theeasmg. i

The instrument casing is formed of two flat, parallel sideplates 11 and12, a flat bottom plate 13 and a flat top plate, 14, FIG. 2, the latterplate being removed in FIG. 1 in order to reveal the instrument opticalsystem. The casing also includes a conventional front bezel assembly 15FIG. 3 comprising a bezel 16 and a planar translucent image viewingscreen 17 of overall rectangular shape. The instrument is illustrated asbeing symmetrical with respect to a longitudinal plane takenperpendicular to the longitudinal axis of the screen and passing throughthe center thereof. As best seen in FIG. 3, the exposed portion of thescreen 17 may include scale indicia appropriately graduated in values ofa measured signal. The screen 17 may take other forms as well. Forinstance, the screen may be divided lengthwise into two halves, one halfbeing opaque and having the scale indicia inscribed thereon and theother half being translucent and displaying the projected vane imageadjacent the scale indicia inscribed on the opaque half. Although thescreen is illustrated as being a planar it may also be arcuate and curveoutwardly. The bezel 16, FIG. 2, is fixed to the front ends of thesideplates 1 1 and 12 by any suitable means. Opposite edges of thescreen 17 are conveniently secured within opposed grooves formed in theinterior walls of the plates 1 1 and 12.

The translucent screen 17 constitutes a light-transmitting surface thattransmits to the front surface of the screen for external viewing vaneimages projected onto the back surface of the screen. Located oppositethe screen 17 is a rear casing wall 18 which mounts an incandescent lamp20 on a pair of metallic clamps 21 which serve to releasably clamp thelamp terminals and additionally, provide part of an external electricalconnection through terminals 22 between the ends opposite of the lampfilament and an undisclosed battery source used to illuminate the lamp.

The filament of the lamp 20 is elongated, the midpoint of the filamentcoincides with the principal axis of a conventional condensing lenssystem and the longitudinal filament axis is perpendicular to thatprincipal axis. The condensing lens system concentrates diverse incidentlight rays received from the lamp filament into a well-defined beam ofvisible light which converges onto that portion of the vane surfacewhich is displaced to intercept the beam. The lens system comprises twocondensing lenses 24 and 25 mounted on base supports 26 and 27,respectively, with the principal axes of the lenses coaligned.

In the illustrated embodiment of the instant invention, the concentratedlight beam emerging from the condensing lens system is disclosed asbeing initially directed toward the screen and then reflected back anddirected toward the rear wall 18 of the instrument casing from whencethe beam is projected for display. The light beam is directed throughthis particular optical path for several practical reasons which willnow be discussed.

Requirements typically imposed upon instruments of this type arefirstly, that the instrument casing have a width, that is, an edgewisedimension, which is limited primarily by the width accorded therectangular viewing screen; secondly, that the instrument casing houseconventional electrical meter movements to drive the vane which may notand typically are not, specifically designed for incorporation in arelatively thin panel instrument and hence may occupy a substantialvolume and thirdly, that the system, and especially the light sourcetherefor, be accessible for cleaning or replacement.

The first requirement is particularly important in those applicationswhere the instrument is to be mounted on an instrument panel in closeside-by-side relationship with other indicating instruments of the sametype. This requirement often arises when it is desired to monitor anumber of different driving or control signals by visually scanning abank of rectangular light bars in side-by-side alignment. To provide awidth of instrument casing equal to, or less than, the width of theviewing screen, the corresponding dimensions of the optical systemutilized by the instrument are similarly limited. Considering now thesecond requirement, it is often preferred that conventional metermechanisms be used to drive the vane. These mechanisms are frequently ofsubstantial size and being opaque, consideration must be given toallocating an area for the mechanism which does not obstruct the opticalsystem. With regard to the third requirement, it is generally preferredthat the incandescent lamp, and possibly in addition, the entire opticallens system be accessible for removal. Since the front portion of theinstrument casing is designed to receive the viewing screen, and withthe instrument mounted in a panel, it is often preferred that at leastthe incandescent lamp be mounted adjacent the rear of the instrument toobtain access thereto from the back of the mounting panel.

The fulfilling of these and other requirements is of course importantfrom a practical viewpoint and is accomplished by the illustratedoptical system. However, it will be understood that it may not beessential for some applications that the convergent light beam bereflected back and then focused onto the viewing screen. For theseapplications, the convergent light beam emerging from the condensinglens system might be focused onto the vane without any interveningreflection.

Considering again the illustrated embodiment of this invention, thelenses 24 and 25, FIG. 1, are positioned to direct the converging beamof light forwardly or toward screen 17, the beam being directed acrossand above the internal surface of the plate 13 and onto a planarreflecting surface 29 of a mirror 30. So that the precise image of thelamp filament does not appear as such on the screen the lenses 24 and 25are mounted so as to be slightly out-of-focus with respect to thatfilament. The perpendicular distance between the principal optical axisof the lens system and the internal surface of this plate 13 isdesignated D1. Ambient conditions permitting, the lenses could bemounted in an aperture formed in the wall 18 and the lamp 20 suitablymounted rearwardly of the lenses and externally of the instrumentcasing. If desired, the entire optical system may be detached from thefront portion of the instrument by having the rear wall 18 and acontiguous rear section of the bottom plate 13 mounting the mirror 30formed of a single piece of metal. Spring clips 31 fixed to the plate 14and the adjoining edge of the plate 13 are exemplary one type offastening means which can be employed to secure the mounting of theoptical system to the front portion of the instrument.

The mirror 30 has an overall rectangular shape and is mounted stationaryon the base 13 by a base support member 31 with the geometrical centerof the mirror substantially concentric with the principal axes of thelenses 24 and 25 and the longitudinal axis of the mirror parallel to thelongitudinal axis of the lamp filament. The light reflecting surface 29of the mirror 30 faces the rear wall 18 and the plane of this surface isinclined with respect to the principal axis of the lens system at anangle equal to the desired angle of reflection. The angle of reflectionis selected so that the concentrated light beam incident to the surface29 is reflected or folded back onto a mirror surface 33 of a rearwardlymounted mirror 34. The angle of reflection is typically on the order of45. The area of the mirror surface 29 is large enough for the mirror toreceive as incident light all of the converging light rays forming theconcentrated light beam. However, the width dimension of the mirror 30,should be small enough so that the upper edge of the mirror 30, FIG. 2,does not obstruct or reflect rearwardly the light which is focused bythe mirror 34 onto the desired portion of the screen 17.

The center of the surface 33 is mounted a second perpendicular distanceD2 from the surface of the plate 13, the distance D2 being sutficientlygreater than the distance D1, so that for some maximum permissiblelongitudinal spacing between the surfaces 17 and 33, prescribed by themaximum permissible instrument casing length, the mirror 34 will focusall incident light onto the desired portion of the back screen surfacewithout the light being obstructed by intervening structures;specifically the upper edge of the mirror 30 and the hereinafterdescribed vane 40 and associated driving means The light beam projectedonto the surface 33 is a diverging light beam having its focal point inthe plane of movement of the vane 40. With the focal point of theconverging lens system in the plane of vane travel, light diffractioneffects, particularly light diffraction at the leading edge of the vanewhich intercepts the focused light beam are minimal, and as a practicalmatter, are not visually detectable on the viewing screen. The vane 40may be driven by any suitable mechanism and is illustrated as beingdriven in an arcuate path by a conventional permanent magnet-movingcoil, or DArsonval, type of electrical instrument movement mountedstationary on the base plate 13. The movement 41 is accommodated betweenthe screen 17 and the mirror 30 so as not to obstruct the instrumentoptical system. The vane 40 is connected to the moving coil of theinstrument 41 by a connecting arm 42. The radius of the arccircumscribed by vane travel may be maximized by positioning themovement 41 relatively close to the screen 17 and by having the lengthof the arm 42 relatively long. lnterference with the light which isprojected onto the screen by the mirror 33 is avoided by mounting theportion of the arm 42 connected to the movement 41 closely adjacent thebottom plate 13, the remaining portion of the arm nearer the vane beingbent upwardly to avoid contact with the free, lengthwise edge of mirror30. The lower edge of the vane is mounted high enough to avoidinterference with the light rays traveling from the lens to the mirror30. Best accuracy is obtained byhaving the pivot axis of the vane andthe principal axis of the optical system in the plane of symmetry of theinstrument. The movement 41 is typically designed such that a driving orcontrol signal applied to the movement 41 causes an angular displacementof the vane which is proportional to the magnitude of that signal.

In order to display the magnitude of the instrument 41 driving signal asa planar rectangular lightbar the vane 40 is also made planar with asimilar and suitably proportioned rectangular configuration. To providea color contrast between the displayed rectangular light bar andcontiguous illumination, the vane 40 is comprised of a transparent colorfilter which transmits an appropriate wavelength or wavelengths of theincident convergent white light. To minimize variations in lightintensity across the surface and along the leading edge 43 of the vane,the vane is mounted on the free'end of the arm 42 perpendicular to thelongitudinal or focal axis of the convergentbeam reflected from themirror 30.

Manifestly, the vane may have configurations other than rectangular,such as triangular, depending upon the configuration which is to beaccorded the illuminated display and as mentioned briefly above may betransparent or colored and/or bear indicium. Moreover, and particularlyin those instances where it is desired to project indicia inscribed onthe vane onto the screen, the vane may be arcuate and concave withrespect to its axis of rotation. The leading vane edge 43 and thatadjoining portion of the vane which intercepts the concentrated beam oflight is projected onto the screen by the concave mirror 34 as abrilliant and sharply defined image. Considering now the details of themirror 34, with the distance between the back screen surface and thesurface 32 fixed by the allowable longitudinal dimension of theinstrument casing and with the distance between the mirror 34 and thevane 40 established at a distance such that the vane will not interceptits own projected image, a radius of curvature for the mirror 34 isselected which will cause the mirror to focus the vane image on the backscreen surface. Inasmuch as the divergent light rays incident to thespherical reflecting surface 33 are not normal to this surface, themirror 34 is tilted forwardly to form an angle of reflection withincident diverging light rays which provides maximum intensity ofillumination to the desired portion of. the viewing screen. Accordingly,the mirror 34 is mounted stationary to the wall 18 by brackets 38 and39, with v the upper edge of the surface33 inclined closer to the screen17 than the bottom edge by an amount which will provide the desiredangle of reflection to the light rays that are reflected forwardly fromthis surface. To obtain a theoretically infinite magnification of thevane image, the plane of vane travel may be made coincident with thefocal point of the mirror 34. Normally, however, this plane is locatedslightly forwardly of the focal point, FIG. 1. The vane end overlyingthe free end of arm 42 may be slotted to permit a limited manual vaneadjustment in directions parallel to the longitudinal axis of the armuntil optimum image sharpness is achieved.

It is important that a concave mirror surface 33 of spherical or of anaspheric shape, for example parabolic, be used to project the vane imageonto the screen because these mirrors are highly efficient lightcollectors and provide an extremely high level of image brightness onthe screen. Moreover, such mirrors do not have the disadvantages ofconventional projection lenses. The mirror 34 is of generallyrectangular shape to conform with the elongated light radiation surfaceof the lamp 20 filament, has a longitudinal axis paraliel to thelongitudinal axis of the lamp 20 and the screen 17 and the length andwidth dimensions of its reflecting surface 33 essentially proportionalto the respective length and width dimensions of the screen portionwhich is to be illuminated thereby. The midpoint of the surface 33 is inoptical alignment with the midpoint of the lamp filament and is in theplane of instrument symmetry.

For reasons related hereinabove, the leading edge 46 of the rectangularlight bar 47, FIGS. 1 and 3, is a bright and sharply defined, magnifiedimage of the vane edge 43. The remainder ofthe bar 47 is similarly abright and sharply definedfmagnified image of the adjoining portion ofthevane which intercepts the concentrated light beam derived from thecondensing lens. The edge 46 forms a sharp, shadow-free interfacebetween it and the projected but unfiltered light rays which are alsofocused onto the screen by the mirror 34. If the vane is formed of atransparent film strip colored to provide an easily recognizable colorcontrast to the background screen illumination, a sharply defined andbrightly contrasting colored light image will appear on the screen whichis devoid of noticeable lightdispersion effects, such as rainbowing. Avisual indication is thereby provided on the screen of the position ofthe vane in the concentrated light beam, the edge 46 moving for example,from right to left as viewed in FIGS. 1 and 3 with increasing values ofsignal applied to the movement 41 and moving from right to left withdecreasing signal values. Obviously, movement 41 could be of a typewhich would drive the vane edge 43 from a point slightly to the right,as viewed in FIG. 1, of the concentrated beam reflected off the mirror30 with one value of measured signal, and leftward from that point withfurther increases in signal values. In the latter case, the projectedleading edge 46 would move correspondingly from left to right as viewedin FIG. 3 with increasing signal values from some initial position whichmay be to the left of the viewing screen and perhaps behind the bezel l6and thusly obscured from view. Other modifications will of course beapparent to those skilled in the art. For example, if it is desired toshape the convergent light beam emerging from the condensing lens systeman opaque mask having an aperture of the desired shape could beinterposed between the vane 40 and the lens 25.

What I claim Is:

1. An indicating instrument comprising, a planar base having a front anda rear end, a rectangular light-transmitting screen portion mounted atthe front end of said base with the light transmitting screen surfacethereof perpendicular to the plane of said base, a source of visiblelight and a lens system mounted adjacent the rear end of said base forcondensing light received from said source into a convergent light beam,said light beam being directed by said lens system across said basetoward said screen, an elongated, concave mirror mounted adjacent saidrear end and having a light reflecting surface positioned to focusincident light onto a portion of said screen, said concave mirror beingdisplaced from said lens system in a direction perpendicular to theplane of said base, a planar mirror interposed between said screen andsaid lens system and including a planar reflecting surface forreflecting the convergent light beam from said lens system, atransparent light filter movable across the reflecting surface of saidplanar mirror in a path, means for mounting said planar mirror at anangle'with respect to the principal axis of said lens system such thatthe convergent light beam is reflected rearwardly to intercept a portionof the path of said filter, means responsive to a control signal fordisplacing said filter in said path, the extent of filter illuminationthrough interception of the reflected light beam being determined by thevalue of the control signal received by the displacing means, saidfilter being comprised of a light-transmitting material selected toprovide a visually detectable color contrast on said screen between theportion of said filter which is illuminated and the noninterceptedportion of the light beam, both portions of the light beam beingreflected back onto said screen by said concave mirror.

2. The instrument as claimed in claim 1, wherein said displaying meansis mounted on said base between said planar mirror and said screen.

3. The instrument as claimed in claim 2, which further comprises, anelongated arm extending over one edge of said planar mirror for couplingsaid filter to said displacing means.

4. The instrument as claimed in claim 1, wherein said light filtercomprises a planar, transparent vane of rectangular shape.

placing means is an electrical measuring instrument movement mounted onsaid base between said screen and said planar mirror and having aportion thereof which rotates through an angle corresponding t themagnitude of the control signal applied thereto, and an elongated armextending over and adjacent one edge of said planar mirror for couplingsaid portion of said instrument movement to said filter to causecorresponding arcuate displacement of said filter.

6. The instrument as claimed in claim 5, wherein said filter iscomprised of a planar, transparent element of rectangular shape.

1. An indicating instrument comprising, a planar base having a front anda rear end, a rectangular light-transmitting screen portion mounted atthe front end of said base with the light transmitting screen surfacethereof perpendicular to the plane of said base, a source of visiblelight and a lens system mounted adjacent the rear end of said base forcondensing light received from said source into a convergent light beam,said light beam being directed by said lens system across said basetoward said screen, an elongated, concave mirror mounted adjacent saidrear end and having a light reflecting surface positioned to focusincident light onto a portion of said screen, said concave mirror beingdisplaced from said lens system in a direction perpendicular to theplane of said base, a planar mirror interposed between said screen andsaid lens system and including a planar reflecting surface forreflecting the convergent light beam from said lens system, atransparent light filter movable across the reflecting surface of saidplanar mirror in a path, means for mounting said planar mirror at anangle with respect to the principal axis of said lens system such thatthe convergent light beam is reflected rearwardly to intercept a portionof the path of said filter, means responsive to a control signal fordisplacing said filter in said path, the extent of filter illuminationthrough interception of the reflected light beam being determined by thevalue of the control signal received by the displacing means, saidfilter being comprised of a lighttransmitting material selected toprovide a visually detectable color contrast on said screen between theportion of said filter which is illuminated and the noninterceptedportion of the light beam, both portions of the light beam beingreflected back onto said screen by said concave mirror.
 2. Theinstrument as claimed in claim 1, wherein said displaying means ismounted on said base between said planar mirror and said screen.
 3. Theinstrument as claimed in claim 2, which further comprises, an elongatedarm extending over one edge of said planar mirror for coupling saidfilter to said displacing means.
 4. The instrument as claimed in claim1, wherein said light filter comprises a planar, transparent vane ofrectangular shape.
 5. The instrument as claimed in claim 1, wherein saiddisplacing means is an electrical measuring instrument movement mountedon said base between said screen and said planar mirror and having aportion thereof which rotates through an angle corresponding to themagnitude of the control signal applied thereto, and an elongated armextending over and adjacent one edge of said planar mirror for couplingsaid portion of said instrument movement to said filter to causecorresponding arcuate displacement of said filter.
 6. The instrument asclaimed in claim 5, wherein said filter is comprised of a planar,transparent element of rectangular shape.